The present invention relates to apparatus for tunneling holes through the ground, as for example air-operated impact devices for tunneling substantially horizontally for the purpose of laying cables or pipes beneath roadbeds or other surface structures.
Air-operated impact devices are commonly used for horizontal tunneling beneath roadbeds or other surface structures and are constructed similar to the device described in my earlier U.S. Pat. No. 4,144,941, issued on Mar. 20, 1979. In the typical operating environment for this type of device, an excavation must be dug to a sufficient depth at the point where the hole tunneling operation is to begin. A second excavation is dug at the desired exit point of the tunneling device, again to a depth sufficient to permit the device to cleanly exit from the ground. After these excavations have been prepared, the tunneling device is carefully placed at the proper depth in the first excavation and is carefully aligned in both a horizontal and vertical plane toward the second excavation. The device is then activated to enter the ground and begin tunneling toward the second excavation. After a period of time, which is dependent upon the depth of tunneling, soil material and condition, length of tunnel and other factors, the device will travel underground in a direction generally aligned with its initial position until it exits from the ground at the second excavation.
Boring devices such as drills, wherein the soil is removed from the ground during drilling, operate differently than impact devices, which form a tunnel by compacting the soil around the device. The hammers of impact devices typically reciprocate at rates approaching sixty times per second, so that each impulse causes a small forwardly directed motion. The tip of the impact device is usually conically shaped, from a narrow front and expanding to a rear dimension which approximates the desired diameter of the hole. As the tip moves forward, the surrounding layer of soil is compacted around the tunnel.
One of the significant problems encountered with the use of impact devices is the length of time necessary to complete the tunnel. This occurs primarily where the soil is highly compacted or dense. In this situation the increased resistance to expansion of the hole delays the forward movement of the device through the soil. The amount and nature of resistance encountered by the impact device also directly affects the operator's ability to direct the impact device in a generally straight line. By decreasing the resistance of the soil to the impact device tip, the impact device will move faster and in a generally straight line toward the second excavation.